From ab0db69a7b11ba1a9dc52d50c94e9e321b70ebf0 Mon Sep 17 00:00:00 2001
From: Sebastiano Tronto <sebastiano@tronto.net>
Date: Fri, 6 Jun 2025 09:13:54 +0200
Subject: Added example 6

---
 06_storage/primes.c | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 06_storage/primes.c

(limited to '06_storage/primes.c')

diff --git a/06_storage/primes.c b/06_storage/primes.c
new file mode 100644
index 0000000..4a17e8d
--- /dev/null
+++ b/06_storage/primes.c
@@ -0,0 +1,94 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+#include <pthread.h>
+
+#include "primes.h"
+#include "storage.h"
+
+#define NTHREADS 16
+#define INDEX(i) ((i) >> 3)
+#define MASK(i) (unsigned char)(1 << ((i) % 8))
+
+unsigned char primes_table[TABLESIZE];
+struct interval { int low; int high; int count; };
+
+void *pthread_routine(void *);
+int primes_in_range(int, int, void (*)(const char *));
+void set_nonprime(unsigned char *, int);
+bool isprime(const unsigned char *, int);
+
+int primes_in_range(int low, int high, void (*log)(const char *)) {
+	static bool table_is_loaded = false;
+
+	pthread_t threads[NTHREADS];
+	struct interval args[NTHREADS];
+
+	if (low < 0 || high < low)
+		return 0;
+
+	if (!table_is_loaded) {
+		if (!read_table(primes_table)) {
+			generate_primes(primes_table, log);
+			if (!store_table(primes_table))
+				log("Error storing table to indexed DB");
+		}
+		
+		table_is_loaded = true;
+	}
+
+	int interval_size = (high-low)/NTHREADS + 1;
+	for (int i = 0; i < NTHREADS; i++) {
+		args[i].low = low + i*interval_size;
+		args[i].high = args[i].low + interval_size;
+		pthread_create(&threads[i], NULL, pthread_routine, &args[i]);
+	}
+
+	log("All threads have started, computing..."); 
+
+	int result = 0;
+	for (int i = 0; i < NTHREADS; i++) {
+		pthread_join(threads[i], NULL);
+		result += args[i].count;
+	}
+
+	return result;
+}
+
+void set_nonprime(unsigned char *table, int n) {
+	table[INDEX(n)] &= ~MASK(n);
+}
+
+bool isprime(const unsigned char *table, int n) {
+	return table[INDEX(n)] & MASK(n);
+}
+
+void generate_primes(unsigned char *table, void (*log)(const char *)) {
+	memset(table, 0xFF, TABLESIZE);
+	set_nonprime(table, 0);
+	set_nonprime(table, 1);
+	for (long long i = 0; i < MAX_PRIME; i++) {
+		if (i % 100000000 == 0 && log != NULL) {
+			char msg[200];
+			sprintf(msg, 
+			    "Could not read table of primes, generating it\n"
+			    "Done %llu / %d", i, MAX_PRIME);
+			log(msg);
+		}
+		if (!isprime(table, i))
+			continue;
+		for (long long int j = 2*i; j < MAX_PRIME; j += i)
+			set_nonprime(table, j);
+	}
+}
+
+void *pthread_routine(void *arg) {
+	struct interval *interval = arg;
+
+	interval->count = 0;
+	for (int i = interval->low; i < interval->high; i++)
+		if (isprime(primes_table, i))
+			interval->count++;
+
+	return NULL;
+}
-- 
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